Synaptosomes prepared from various regions of rat brain are used as models of nerve endings to study mechanisms of uptake and release of neurotransmitters in the central nervous system. The distribution of neurotransmitters within synaptosomes are determined by hypotonic shock on millipore filters following neurotransmitter uptake or uptake plus drug-induced release. Depolarizing levels of K ion releases norepinephrine and serotonin from both the shock resistant and shock susceptible fraction. BIBLIOGRAPHIC REFERENCES: Dratman, M.B., Crutchfield, F.L., Axelrod, J., Colburn, R.W. and Thoa, N.B.: Localization of tri-iodothyronine in nerve ending fractions of rat brain. Proc. Natl. Acad. Sci. 73: 941-945, 1976. Ho, A.K., Ng, L.K., Thoa, N.B., and Colburn, R.W.: In Vitro Interactions on Synaptosomal Uptake of Morphine and Biogenic Amines. In Ford, D. and Clouet, D. (Eds.): Tissue Responses to Addictive Drugs. New York, Spectrum, 1976, pp. 209-218.